It is often desirable to know absolute values for optical properties of atmospheric aerosol particles. For example, it may be beneficial to know the absolute aerosol backscatter coefficient of aerosol particles in a section of interest in the atmosphere. The value of an absolute aerosol backscatter coefficient represents the ability of atmospheric aerosol particles in a particular section of atmosphere to scatter optical energy propagating through that particular section of atmosphere. Since an absolute aerosol backscatter coefficient is obtained by calibrating the lidar system with respect to molecular scattering and applying that calibration factor to the aerosol backscatter detected, the resulting value for the absolute aerosol backscatter coefficient is calibrated. Therefore, absolute aerosol backscatter coefficient measurements, as opposed to measurements of uncalibrated, relative aerosol backscatter intensity, allow for direct comparison of measured absolute aerosol backscatter coefficient values from differing locations and dates. The absolute aerosol backscatter coefficient data can also be expressed with units, such as per meter per steradian.
The remote identification of absolute aerosol backscatter coefficients for significant sections of the atmosphere is critical in many applications. One such area is the remote measurement of biological, chemical or radiological agents in the atmosphere. The potential of intentional releases of biological, chemical or radiological contaminants into the atmosphere is a real and serious threat to civilian populations in the United States and abroad and also to military personnel throughout the world. It would be beneficial to be able to determine and monitor the absolute aerosol backscatter coefficient of a plume of suspected or known harmful aerosol particles. This information could be used to help identify particle concentration and diffusion characteristics. This would be valuable information in standoff detection and homeland security situations where the information could be used to predict the lethality of a pathogenic aerosol cloud.
Absolute aerosol backscatter coefficients can be used to determine the optical thickness of aerosol particles in a section of atmosphere. Also, particle concentration can be obtained when measuring a plume consisting of particles of an identified type if the size distribution and scattering efficiency of that type of aerosol particle is known. This data can be valuable to atmospheric researchers and modelers attempting to determine the climatic effects of atmospheric particles. For example, scientists attempting to determine the effects of pollution on global warming would find real-world absolute aerosol backscatter coefficient data valuable in modeling current and future effects of pollution. The data could also be coupled to corresponding climate conditions present when the measurements were taken. Scientists would also find valuable the ability to monitor the effects and particle distribution of large-scale natural events such as dust storms, forest fires and volcanic eruptions by determining absolute aerosol particle backscattering coefficients in various locations after such events.
High Spectral Resolution Lidar (HSRL) is a known technique in the lidar community for determining absolute aerosol backscatter coefficients. However, a number of factors limit the performance of known HSRL systems. One such limitation is that known HSRL systems operate at relatively short wavelengths. These short wavelengths correspond to wavelengths where the human eye is particularly vulnerable to injury. Therefore, extreme care must be taken when operating an HSRL system at such a wavelength.
Generally, there are two ways in which current HSRL systems operate. First, some current systems transmit relatively weak pulses to operate in an eye-safe manner. The relatively weak pulses in turn require these systems to time average backscatter readings over a relatively long period of time resulting in low temporal resolution and the lack of ability to scan significant sections of the atmosphere rapidly. This lack of ability to scan rapidly limits the ability of these types of HSRL systems to track large atmospheric structures, such as plumes of aerosol particles.
Second, some current systems transmit higher pulse energies at the aforementioned relatively short wavelengths and do not operate in an eye-safe manner. Operation in this manner requires extraordinary safety precautions, such as verifying that the intended path of the transmitted beam is clear of any humans, such as pilots in passing planes, who could be harmed by the beam. This severely limits the use of these systems and prevents these systems from operating in a continuous, unattended fashion.